I use Drupal and Magento quite a bit and am able to automate the creation of users to csv files on those domain.

It might be quite good if we could specify a remote location in the Sendy backend where such a csv file lives. Then when Drupal (et al) update that file with new users cron could go fetch it and process it.

I guess that you are right, but my problem stems from the fact that I have a good understanding of how drupal modules work, but very little expertise with programming in php. Which means that integrating your api is beyond me!